Insufficient sleep is a major health and safety problem for young and older adults alike. National polls indicate that more than a quarter of adults 18-84 years of age regularly sleep less than 7 h per night, with 15% reporting sleeping less than 6 h per night. The deficits in alertness and performance ability associated with insufficient sleep can lead to greater risk for automobile accidents, home and workplace accidents, and on-the-job errors in sleepy individuals. Insufficient sleep can also have a negative effect on the quality of life, with impacts on irritability, mood, and motivation. There is evidence from acute sleep deprivation studies that older adults may respond differently to sleep loss than young adults, but studies comparing the response to chronic sleep restriction in older and young adults have not yet been conducted. Here, we propose to investigate the consequences of 4 weeks of chronic insufficient sleep on daytime alertness and performance in older (55-70) and young (18-30) adults. Subjects will be scheduled to sleep an equivalent of 5 h per 24 h in a forced desynchrony protocol, in order that we can investigate both the circadian and wake-dependent contributions to alertness and performance across the 4 weeks of sleep restriction. We will use subjective and objective measures to assess alertness across each waking episode, and subjects will perform a battery of tasks several times per day to measure the consequences of sleep restriction on cognitive throughput, sustained attention, reaction time, motivation, mood and short-term memory. Our study design will allow us to assess independently the impact of time-of-day (circadian phase) and duration of prior wakefulness, as well as their interactions, on measures of daytime alertness and performance under conditions of sleep restriction across 4 weeks. Results from this study have important implications for understanding the consequences of chronic insufficient sleep in older adults. Most adults regularly fail to get sufficient sleep, but few studies have attempted to quantify the consequences of chronic insufficient sleep in middle-aged and older people. Insufficient sleep can have important effects on health, safety, and quality of life. Knowledge of how the alertness, performance, and memory of healthy older people respond to insufficient sleep will provide a basis for future research and for advice and recommendations for maintaining health and well-being as we age.